The construction and use of long-distance transcontinental pipelines for transmission of liquid and gaseous hydrocarbon fuel stocks is rapidly increasing throughout the world. Such pipelines are used to convey crude liquid energy products from remote terrestrial and deep-sea drilling sites to refineries, from where extensive long-distance piping infrastructures are used to deliver refined liquid energy products to urban and/or industrial areas for redistribution by local piping infrastructures or overland by tank trucks. Long-distance piping infrastructures are also used for transcontinental conveyance of natural gas and liquefied natural gas. As world energy demands increase, the pace of construction of such pipelines is expected to increase. United States, for example, has over 400,000 miles of natural gas delivery pipelines in place and is expected to increase this infrastructure by 50% during the next twenty years (Sivathanu, Technology Status Report on Natural Gas Leak Detection in Pipelines prepared for the US Dept. of Energy, Contract No. DE-FC26-03NT41857).
There are significant and potentially catastrophic human and environmental safety risks associated with leakage of such hydrocarbon fuel stocks from long-distance transmission pipelines as a consequence of pipe cracking due to materials fatigue, to defective joints, and accidental or deliberate physical encroachments and resulting damage. Consequently, a wide variety of methods are employed to routinely inspect and monitor these piping infrastructures for leakage. Above ground liquid transmission pipelines can be monitored by visual inspections from low-flying aircraft while gas pipelines can be monitored by aircraft equipped with infra-red sensing instruments. Another method for monitoring long-distance pipelines for leakage is to measure differences in flow-volumes between adjacent pumping stations. Satellite-based hyperspectral techniques are also used for routine monitoring. Such methods are typically suited for detection of large leakages and therefore are more useful for spotting and locating significant pipeline breaks and are not capable of detecting small leaks. Another problem with such monitoring methods is that the detected leakages must be confirmed by on-site visual inspections, Furthermore, such overhead monitoring methods are not useful for monitoring leakage from underground transmission piping or for transmission piping connecting deep sea drilling rigs to land-based depots and refineries.
Considerable efforts have been placed during the past two decades on the development of passive leakage-sensing devices that are based on: (a) the use of sound-wave systems, light-wave systems, combination heat & light-wave systems that transmit signals between signal sources and signal receivers, and (b) the detection and measurement of physical interference by leaked materials with the transmission of the signals within these types of systems. Such passive leakage-sensing devices are typically configured as cable systems which are placed adjacent to or in very close proximity to pipelines. The transmission of signals and any interferences with signal transmission are constantly monitored by remote signal processing equipment and instruments. Examples of such systems currently in use include acoustic monitoring systems, millimeter wave radar systems, infrared thermography, and various fiber-optic systems such as single-wave continuous-emission laser systems, single-wave pulsed laser systems, multi-wave pulsed laser systems, and distributed temperature sensing (DTS) optical systems.
Many light-wave-based systems typically comprise an optical time domain reflectometer (OTDR) which includes a light source for emitting pulsed signals. The OTDR is an instrument commonly used in the fiber optics industry for receiving and analysis of back-reflected light signal transmissions. The systems generally function by the emission of a pulsed laser light source into and subsequent transmission along a fiber optic waveguide. The light energy is reflected within the fiber optic waveguide back toward the source, but is diverted by a beam splitter which typically redirects about 50% of the reflected light energy into the OTDR's detection and recording section wherein the intensity of the reflected light is measured and recorded. Any interference with the flow of light energy along the fiber optic cable will be detected and measured by the OTDR. Significant changes in OTDR values when compared to reference values (i.e. previous measurements of reflected light), can be used to identify leakage events. The advantages of such passive leakage-sensing devices include relatively inexpensive to install, they don't interfere with the operation of pipelines, and can be continuously monitored by mobile or fixed stations. However, the disadvantages inherent with these types of passive leakage-sensing devices is that the systems typically have a high rate of false alarms, and that the instrumentation, processing and staffing required for continuous monitoring are expensive to install and operate. Although fiber optic light-based systems are useful for detecting occurrence of leakages in pipelines, it is difficult with the current state-of-the art systems, to precisely pinpoint pipeline leakage locations within long-distance ranges. Discrete fuse-block technologies have been developed to cut or bend light transmission along fiber optic waveguides in response to pipeline leakage; see for examples UK Patent Application No. GB 2 100 420 A, and MacLean et al., 2003 In Sensors and Actuators A, vol. 109, pages 60-67. However, such fiber optic leakage-sensing devices are expensive to manufacture, are difficult to replace after leaking pipe sections have been repaired or replaced, and do not enable precise identification of leakage locations, i.e., to within 1 meter or less, unless the sensing devices are installed at spacings of 1 m or less which will significantly increase the cost of the detection system.